Time delay fuse



Dec. 11, 1956 v, sUNDT ET AL TIME DELAY FUSE Filed April 28, 1954 INVEN TOR Jd wmw afia g United States Patent i 2,773,960 Till/1E DELAY FUSE Edward V. Sundt, Wilmette, and Hugh A. Triplett, Des Plaines, Ill., assignors to Sundt Engineering Company, Des Plaines, 111., a corporation of Illinois Application April 28, 1954, Serial No. 426,097 14 Claims. (Cl. 200-123) The principal object of this invention is to provide an improved time delay fuse for electric circuits WhlCll provides substantially 1nstantaneous action upon the occurrence of high overloads, which provides a time delayed action upon lower sustained overloads, which prevents needless fuse blowing on harmless overloads, which 1s of a new and improved construction, which is foolproof in operation, and which can be inexpensively manufactured.

Basically, the time delay fuse of this invention is a heat operated device wherein heat is generated therein through the passage of electric current and absorbed over time up to a calibrated point of fusion in order tosecure the desired delay. Two physical factors are utilized to secure the time delay, the specific heat of the entire fuse element and the latent heat of fusion of a body of fusible metal purposely added to the element. 7

Briefly, the time delay fuse of this invention includes an electrically conductive fuse element formed from a fit fusible metal and having between its ends a receptacle 7 portion and a resistance heating portion of limited cross sectional area adjacent the receptacle portion. A body of fusible metal of lower melting point than that of the fuse element is secured in the receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity therebetween. Electrical terminals are connected to the ends of the fuse element for incorporated the same in an electric circuit to be protected.

A sustained overload in the electric circuit causes the resistance heating portion of the fuse element to heat the fuse element and the body of fusible metal substantially as a unit. The heating of the resistance heating portion of the fuse element to the melting point thereof for breaking the electric circuit being protected is delayed by the specific heat and the latent heat of fusion of the body of fusible metal. In this way substantial time delaysun blowing of the fuse at the resistance heating portion thereof may be obtained. At the same time a high overload, such as caused by a short circuit, will substantially instantaneously blow the fuse at the resistance heating portion thereof. I

The fuse element of the time delay fuse of this invention is formed from a fusible metallic wire having between its ends a substantially tubular coiled wire receptacle portion and the uncoiled portions of the wire forming the resistance heating portions. Preferably, the fuse element includes between its ends a pair of coiled wire receptacle portions, an inner wire resistance heating portion anda pair of outer wire resistance heating portions. Here, the

2,773,950 Patented Dec. 11, 1956 The bodies of fusible metal may be secured in the coiled wire receptacle portions merely by dipping the fuse element in a molten pool of the lower melting point fusible metal. Substantially any kind of fusible metals may be used for the fuse element and the fusible bodies depending upon the fusing conditions to be met. They may be basic metals or alloys of metals, eutectic alloys being preferred for the lower melting point fusible bodies since their melting conditions may be more readily determined and calibrated. Preferably, the outer end turns of the coiled wire receptacle portions are spread from the remaining turns thereof and are free from the body of fusible metal to compensate for thermal expansion and contraction. Also, the outer end turns of the coiled wire receptacle portions have a larger diameter than the remaining turns thereof for centering the fuse element in the tubular enclosure.

Further objects of this invention reside in the detail of construction of the time delay fuse and in the cooperative relationships between the component parts thereof.

Other objects and advantages of this invention will become apparent to those skilled in the art upon reference to the accompanying specification claims and drawing in which:

Fig. l is a perspective view of a time delay fuse constructed in accordance with this invention;

Fig. 2 is a vertical sectional view taken substantially along the line 22 of Fig. 1;

Fig. 3 is an enlarged sectional view of a part of the fuse element;

Fig. 4 is a view looking from the right of Fig. 3;

Fig. 5 is a view similar to Fig. 3 but illustrating a body of fusible metal secured therein;

Fig. 6 is a view looking from the right of Fig. 5.

The time delay fuse of this invention is generally designated at 10. It includes an electrically conductive fuse element 11 formed from a fusible metallic wire. Between its ends the fusible metallic wire is coiled into a pair of spaced apart substantially tubular coiled wire receptacle portions 12 and 13. The wire extends between the coiled wire receptacle portions 12 and 13 and forms a resistance heating portion 14. The ends of the wire extend outwardly from the outer ends of the coiled wire receptacle portions 12 and 13 and form outer resistance heating portions 16 and 17. The fabrication of the fuse element 11 into the form illustrated may be carried out by usual wire forming machinery. The outer end turns of the coiled wire receptacle portions 12 and 13 are separated from the remaining turns thereof as indicated at 18 and also these outer end turns 18 are of larger diameter than the remaining turns.

Secured within the receptacle portions 12 and 13 are bodies 22 and 23 of fusible metal having a lower melting point than that of the fuse element. These lower melting fusible metallic bodies 22 and 23 are readily secured in the receptacle portions 12 and 13 by first dipping the fabricated fuse element in a fiux, then dipping the fuse element in a molten pool of the lower melting point fusible metal wherein the fusible metal enters into the receptacle portions 12 and 13 and is maintained therein by capillary attraction, and then rinsing the assembly to remove excess flux. In this way the bodies 22 and 23 of fusible metal are secured in the receptacle portions 12 and 13 in intimate contact therewith so as to provide maximum thermal conductivity therebetween. Because the end turns 18 of the coiled wire receptacle portions 12 and 13 are separated from the remaining turns thereof they are free from the bodies 22 and 23 of the fusible metal so that they may readily bend.

The fusible element may be encased in a tubular closure 25 formed of electrical insulating material such as glass. Preferably the ends of the tubular closure 25 are formed latent heat of fusion.

inwardly as indicated at 26 and 27. "Received'over'the ends of the tubular closure are metallic caps 28 and 239, these caps being secured in place and being electrically connected to the ends of the fuse element ill by means of solder indicated at Th and 31. Accordinglythe caps 28 and 25" act as electrical terminals for the fuse element "for connecting the same in an electric circuit to be protected. Because the endturns 18 of the'receptacle portions 12 and 13 are of'larger diarneterthan the remaining turns they effectively operate to center the fuse element in the closure member 25. Because the end turns 18 are separated from'theremaining'turns of thecoiled wire receptaclep'ortions Hand '13 they may readily flex so as to compensate"forthernral'eirpansion and contraction of the fuse element.

A sustained overload "in the"elec-tric circuit being'protected causes the resistance heating portions Mild and 17 of the fuse element to heat the 'fuse element and the bodies22 and 23 of fusible metal 'substantiallyas'a unit. The heating of the resistance heating portions of the fuse element tothemeltingpointtliereof'for breaking the electric circuit being protected "is' delayed by thespe'cific heat of the unit as awholeandthelatentheat effusion of the'bo dies of fusible metal. In this way substantial timeidelays in blowing of the fuse at'the resistance heatin'g-p'ortions thereof may be obtained. At the 'same'time a high overload, such as caused by a short circuit, will substantially instantaneously blow the fuse at the resistance heating portions thereof. The delayed blowing of the fuse normally takes place'at the innerresistanceheating portion 14, where blowing conditions maybe more eterfof .040 inch may be utilized and here the number of turns for each "receptacle portion 12 and 13 would 'oel 'about seven and one-half turns. The inside diameter ofjthe receptacle portions 12 andlS would be substantially,.075 in'cln Here also the fusible metal forming the bodies22 and'23 would be eutectic of. tin and lead, 63% "tinand 37% 'lead. Of course other kinds of fusible metals "may be used. in practice it is preferable to utilize a fusible metaljnot having a melting point less than 288 R, which is the eutectic of'tin' and bismuth, nor is it desirableto have'lthe" maximum melting point more than 690 F., 'which is thecutectic of cadmium and silver. The eutectic of tin and lead, rnelting at approximately 360 F.'is a fit om romi e ternperaturev and ishigh enough abovenorj ntalambient temperatures that the blowing characteristics 'of the fu'sesjare not influenced greatly thereby.

Injoperation in service, the assembled fuse will carry 110% of rated current for a minimum of fourfhours.

I f the current is increased to any value from about 135% of rating and up to'less than 409% of rating, theresis' tance portions of thev fuse element generate enough heat to bring 'the'metal in the receptacle portions up to the melting points thereof. Up to this point the delay in the fuse action is due principallyto the specific heat of the fusible metal'bodies. Above this point'the fusible metal is"'chan'ged from solid to liquid and no further temperature rise takes place until the fusible metal has changed state and a further delay is thus obtaineddu'e to the epending on the alloy used this delay is several times that obtained by'the specific 'heat only. At 135% of rating, the fuse will blow inabout one'hourjwhile at 400% rating, it Will blow in substantially five seconds. Above 400% rating the fuse willblow at a time inversely proportional tothe overload,

Whilefor purposes of illustration one-form-- of-fthis invention has been disclosed other forms thereof may become apparent to those skilled in the art upon reference to this disclosure and therefore this invention is to be limited only by the scope of the appended claims.

We claim as our invention:

1. A time delay fuse comprising an electrically conductive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high melting point-and'having between its ends a substantially tubular coiled wire receptacle portion and a wire resistanceheating portion at each end of and adjacent to the receptacle portion, a body of fusible metal having a predetermined relatively low melting point and secured in the receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity therebetween,and electrical terminals connected to the endsof the fuse element for incorporatingthe same in an electric circuit to be protected, a sustained overload --in the-electric circuit causing the resistance heating portions of the fuseelement to heat the fuse element-and the body of fusible metal substantially as a unit, and. the

heating ofthe resistance heating portions of the fusc elcment to the relatively high point thereof being delayed by the specific heat and the latent heat of fusion of the body of relatively low melting point fusible metal.

2. Atime delay fuse comprising electrically conductive singlepiece'fuse-element formed from a fusible metallic wire having a predetermined relatively high melting point and having between its ends a pair of spacedv apart substantially tubular coiled wire receptacle portions, an inner wire resistance heating portion extending between-the receptacle portions and an outer wire resi'stanceheating portion extending from the outer ends of thereceptacle portions, a body of fusible metal having a predetermined relatively low melting point and secured in each receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity 'therebetween, and electrical terminals connected to the ends of'the fuse element for incorporating the same in an electric 'circuit to be protected, a sustained overtive :singlepiece fuse elementformed from a fusible metallic wire having a predetermined relatively high smelting point-and'havingbetween its ends a substantially tubular coiled wire'receptacle portion and a wire resistance'heating portion at each end of and adjacent to thereceptacle portion, a body of-fusiblc metal having. a

- predetermined relatively low melting point and secured in the receptacle portion of the fuse element in intimate contact-therewith by dipping the fuse element in a molten pool of said lower melting point fusible metal so as to provide high thermal conductivity therebetween, and electrical terminals connected to the ends of the fuse element' for. incorporating the same in an electric circuit to be protected, a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to heatthe'fuse-element and the body of fusible metal substantially as a unit, and the heating of the resistance heating portions of the fuse element to therelatively high melting point thereof being delayed by the specific heat and the latentheat of fusion of the body of relatively low melting point fusible metal.

4. A time delay fuse comprising an electricallyconduc- 'tive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high melting point and having between its ends a pair of spaced apart substantially tubular coiled wire receptacle portions,

an inner wire resistance heating portion extending between the receptacle portions and an outer wire resistance heating portion extending from the outer ends of the re ceptacle portions, a body of fusible metal having a predetermined relatively low melting point and secured in each receptacle portion of the fuse element in intimate contact therewith by dipping the fuse element in a molten pool of said lower melting point fusible metal so as to provide high thermal conductivity therebetween, and electrical terminals connected to the ends of the fuse ele 'ment for incorporating the same in an electric circuit to be protected, a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent heat of fusion of the bodies of relatively low melting point fusible metal.

5. A time delay fuse comprising an electrically conductive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high melting point and having between its ends a substantially tubular coiled wire receptacle portion and a wire resistance heating portion at each end of and adjacent to the receptacle portion, a body of fusible metal having a predetermined relatively low melting point and secured in the receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity therebetween, a tubular closure of electrical insulating material for the fuse element, electrically conductive caps secured to the ends of the tubular closure and forming electric terminals for connecting the fuse in an electric circuit to be protected, means for electrically connecting the ends of the fuse element to the caps, a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to heat the fuse element and the body of fusible metal substantially as a unit, and the heating of the resistance heating portions of the fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent heat of fusion of the body of relatively low melting point fusible metal.

6. A time delay fuse comprising an electrically conductive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high melting point and having between its ends a pair of spaced substantially tubular coiled wire receptacle portions, an inner wire resistance heating portion extending between the receptacle portions and an outer Wire resistance heating portion extending from the outer ends of the receptacle portions, 21 body of fusible metal having a predetermined relatively low melting point and secured in each receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity therebetween, a tubular closure of electrical insulating material ffor the fuse element, electrically conductive caps secured to the ends of the tubular closure and forming electric ,terminals for connecting the fuse in an electric circuit to \be protected, means for electrically connecting the ends of the fuse element to the caps, a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to heat the fuse element and the bodies of fusible metal as a unit, and the heating of rthe resistance heating portions of the fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent heat of fusion of the bodies of relatively low melting point fusible metal.

7. A time delay fuse comprising an electrically conductive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high melting point and having between its ends a substantially tubular coiled wire receptacle portion and a wire resistance heating portion at each end of and adjacent'to the therebetween, atubular closure of electrical insulating material for the fuse element, electrically conductive caps secured to the ends of the tubular closure and forming electric terminals for connecting the fuse in an electric circuit to be protected, means for electrically connecting the ends of the fuse element to the caps, the end turns of the coiled wire receptacle portion being spread from the remaining turns thereof and being free from the body of fusible metal to compensate for thermal expansion and contraction, a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to heat the fuse element and the body of fusible metal substantially as a unit, and the heating of the resistance heating portions of the fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent heat of fusion of the body of relatively low melting point fusible metal.

8. A time delay fuse comprising an electrically conductive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high melting point and having between its ends a pair. of spaced apart substantially tubular coiled wire receptacle portions, an inner wire resistance heating portion extending between the receptacle portions and an outer wire resistance heating portion extending from the outer ends of the receptacle portions, a body of fusible metal having a predetermined relatively low melting point and secured in each receptacle portion of the fuse element intimate contact therewith so as to provide high thermal conductivity therebetween, a tubular closure of electrical insulating material for the fuse element, electrically conductive caps secured to the ends of the tubular closure and forming electric terminals for connecting the fuse in an electric circuit to be protected, means for electrically connecting the ends of the fuse element to the caps, the outer end turns of the coiled wire receptacle portions being spread from the remaining turns thereof and being free from the bodies of fusible metal to compensate for thermal expansion and contraction, a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to heat the fuse element and the bodies of fusible metal as a unit, and the heating of the resistance heating portions of the fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent heat of fusion of the bodies of relatively low melting point fusible metal.

9. A time delay fuse comprising an electrically conductive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high melting point and having between its ends a substantially tubular coiled wire receptacle portion and a wire resistance heating portion at each end of and adjacent to the receptacle portion, a body of fusible metal having a predetermined relatively low melting point and secured in the receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity therebetween, a tubular closure of electrical insulating material for the fuse element, electrically con.- ductive caps secured to the ends of the tubular closure and forming electric terminals for connecting the fuse in an electric circuit to be protected, means for electrically connecting the ends of the fuse element to the caps, the end turns of the coiled wire receptacle portion having a larger diameter than the remaining turns thereof forcentering the fuse element in the tubular closure, a sus-- tained overload in the electric circuit causing the resistanceheating portions of the fuse element to heat the fuse element and the body of fusible metal substantially as a. unit, and the heating of the resistance heating portions:

of the fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent' heat of fusion of the body of relatively low melting point' fusible metal.

l0. A-time delay fuse comprising an electrically conductive single piece fuse element formed from a fusible metallic wire having a predetermined relatively high; melting point and having between its ends a pair of spaced.

apart substantially tubular coiled wire receptacle "portions, an inner wire resistance heating portion extending-between the'receptacle portions "and'anouter' Wire resistanceheating portion extending from theouter ends of the receptacle portions, a body of'fusible metal having a predetermined relatively low melting point and secured'in each receptacle'portion of the fuse element in'intimate contact therewith so as to provide high thermal conductivity therebetween, a tubular closure of electrical insulating materialfor the fuse element, electrically conductive caps secured to the'ends of the tubular closure and forming electric terminals for connecting the fuse in an electric circuit to be protected, means'for electrically connecting the ends-of the fuse element-to the cap s,-theouterend turns of the coiled wire receptacle portions having a larger diameter than the remaining turns thereof 'for centering the -'fuse-element in-the tubular closure, a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to heat the fuse element and the bodies of "fusible metal as a unit, and the heating of the resistance heating portions of the fuse element to the relatively high melting point thereof being delayed by the specific heat'and the latent heat-of fusion of the bodies of relatively low melting point fusible metal.

11. Annie delay fuse comprising an electrically cn ductive single piece fuseelementformed'fromafusible metallic wire having a predetermined relatively high melting point and having between its ends a substantially tubular coiled Wire receptacle portion and a wire resistance heating portion'at each end of and adjacent to the receptacle portion, a body of fusible metal having a predetermined relatively low melting point and secured in the receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity therebetween, a tubular closure of-electrical insulating material for the fuse element, electrically conductive caps secured to the ends of the tubular closure and forming electric terminals for connecting the fuse in an-electric circuit to be protected, means for electrically connecting the ends of the fuse element to the caps, the end turns ofthe coiled wire receptacle portion being spread from the remaining turns thereof and being free from the body offusiblc metal to compensate for thermal expansion and contraction, the end turns of the coiled Wire receptacle portion having larger diameter than the remaining turns thereof for centering the fuse element in the tubular closure a sustained overload in the electric circuit causing the resistance heating portions of the fuse element to heat the fuse element and the body of fusible metal substantially as a unit, and the heating of the resistance heating portions of the fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent heat to fusion of the body of relatively low melting point fusible metal.

12. A time delay fuse comprising an electrically conductive singlepicce fuse element formed from a fusible metallic wire having a predetermined relatively high meltingpoint and having .between its ends a pair of spaced apart substantially tubular coiled wire receptacle portions, an inner wire resistance heating portion extending between the receptacle portions and an outer wire resistance heating portion extending from the outer ends of the receptacle portions, 21 body of fusible metal having a predetermined relatively low melting point and securedin eachreceptacle portion .of the fuseelement in intimate contact therewith so as to provide high thermal conductivity .therebetween, a tubular closure ofelectrical insulating material forthe fuse element,.electrically conductivecaps secured tothe ends ofthe tubular closure and forming electric terminals for connectingthe fuse in. anrelectric circuit-to be protected, means for-electrically connecting theendszofpthefuseelementto the caps, 'the outer 'endxturns of the coiled wire receptacle portions qv being spread. from. the :remaining .rturns .thereof and :being-:free-1from the bodies of 'fusible metal to compensate cfor thermalv expansion and contraction, the outersend turnsof the coiled wire receptacle portions having ;-a i larger diameter; than the remaining turns thereof for :centering the fuse element in the tubular closure, a sustainedoverload in the electriccircuit causing themsistance heatingportions of thefuse element to'heatithe fuse:,element and the bodies of.fusib1e metal as a unit, and 'the heating'of-the resistance heating portions ofthe fuse element to the relatively high melting point thereof being delayed by the specific heat and the latent heat of fusion of the bodies: of relatively low melting, point fusible metal.

13; A: time delay fuse'comprising-an electricallyconductive single:piece fuse element formed fromJ-a fusible metallic wire having-a predetermined relatively high melting point and'having' between its ends a substantially tubularcoiled-wire receptacle portion. and a wire :resistance heating portion at each end of and'adjacent to the receptacle portion, a body of fusible metal having'a predeterminedzrelatively low melting point. andsecured in the receptacle portion of the fuse element in intimate contact therewith so asto provide high thermal conductivity therebetween, and electrical terminals connected to the ends of the fuse element for. incorporating the same in an" electric circuit to be protected, the end-turns of the coiled wire receptacle portion being spread-from the remainingturns thereof and being free from the body of fusible metal to compensate for thermal expansion and contraction, a sustained overload in the electric circuit causi-n-gtherresistance heating portions of'the fuse ele rnenttoheat-the fuse element and the body of fusible metal substantially as a unit, and the heating of theresistance:heatingportions of the fuse element to the relativelyhigh'melting point thereof being delayed by the specific heat and the latent heat of fusionof the body of relatively low melting point fusible metal.

14. A time delay fusecomprising an electrically conductivesingle,piece-fuse element formed from a fusible metallic Wire having a predetermined relatively high melting point and-having between its ends a pairof spaced apart substantially tubular coiled wire receptacle portions, anin-ner'wire resistance heating portion extending between the receptacle portions and an outer wire resistance heating portion extending from the outer ends of the receptacle portions, abody of fusible metal having a predetermined-relatively low melting point and'secured in each receptacle portion of the fuse element in intimate contact therewith so as to provide high thermal conductivity therebetween, and electrical terminals connected to the-ends of'the fuseelement for incorporating the same in an electric circuit to be protected, the outer end turns of the coiled wire receptacle portions being spread from the-remaining turns thereof and being'free from the;

bodies of' fusible metal to compensate for thermal ex-," pansion and contraction, a sustained overload in the, electriccircuit causing'the resistance heating portions of;

thefuse element to heat. thefuse element and 'the bodies offusible metal as a'unit, and the heating of theresistance heating portions of thefuse element to the rela tivelyhigh melting point thereof being delayed by the spec'ificheatand the latent. heat of fusion of the bodies of relatively low melting point fusible metal.

References Cited in the file of this patent UNITED STATES PATENTS 

